If you can, every time you feel the need for drugs/alcohol whatever, go for a walk on the beach.
It’s only simple thing and it might only make it a bit better for a small amount of time, but it helps.
I’ve lived in Aus my whole life and no matter how good or bad I feel, the beach always makes it better.
Good luck, and don’t beat yourself up for the drugs/sleeping around. We’ve all got to deal with things as best we can.
Small steps.

Why do you need a “career” as such? I get the student loans are a pain in the ass, but maybe do the trip thing, get a job as a bartender or something. Don’t be afraid to make change because you feel obliged to stick to a certain path.
Yep, maybe business school was a waste, might be hard to accept, but once you do and focus on where you want to go, not what’s holding you back, there might some indication of where that is. Even if you have to stick it out for another 6 months – a year to save some money – you have a goal in mind, something actionable, rather than the constant wondering you might feel better.

This is tricky and I’ve been through it before myself (although i was the negative one).
One of the things that worked for me and my BF (we’re still together) was to each write a list of:
What we wanted out of the relationship (like, go on date nights, take trips, live overseas etc)
What I wanted to do more of as an individual (so things you may have been ignoring after a while… an old hobby, certain friends etc – we both wrote things we wanted to do individually and spend more time on).
Then, things we wish the other person would do (nothing totally changing – but noting some of the areas you would like to see a little change. For me I wanted my partner to be a bit more proactive in our social lives, as I was always the one planning trips and parties etc. and it was getting boring always been told yes to everything. There were some others but that’s one example) He wanted me to stop bringing up stuff from the past, basically learn to let go of old issues and in a sense, have the same laid-backneed as I did when we first met)
We then both signed the lists (corny I know, but it worked for us). Then we put the lists away (after a good read of each others and ours), and re-assess in 3 months time.
It gave us both things to work on and an actual timeline to measure it by. Because we wrote it down to begin with, it was clear and we both got our say, not an off-hand comment followed by interruptions and in inevitably miscommunication.
It might not be for you, but if nothing else, you’ll get an insight into what he’s thinking and if you’re priorities/goals line up.
Good luck!